Merging using only fast-forward

Hi,

I have a work flow where A is only allowed to rebase and B is only allowed to fast-forward. They both fetch from each other and rebase/merge using pull. On A I can use the configuration branch.<name>.rebase but it does not seem to be an option to enforce a fast-forward on B.

If there are changes on both A and B that have not yet been integrated (by A doing a pull (fetch/rebase)) I want a pull (fetch/merge) on B to fail.

It would be useful with a configuration ala branch.<name>.fast-forward or an option --ff-only to git-merge so I could use the configuration branch.<name>.mergeoptions.

Is there any other way to achieve this?

Thanks for a great tool. I am using git with accurev where B is the integrator between git and accurev (commits flow in both directions). I imagine this could be useful in other environments as well.